A passive optical network (PON) consists of one or more optical line terminations (OLTs) (each of which is a service provider node), a number of optical network units (ONUs) (each of which is a subscriber node), and the fibers and splitters between them (called an optical distribution network (ODN)). One type of PON is a wavelength division multiplexing (WDM) PON, which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (colors) of laser light.
A WDM PON can incorporate an arrayed waveguide (AWG) within its ODN. When laser light enters an AWG from a single fiber, the laser light is de-multiplexed into multiple separate wavelengths with each wavelength exiting a port of the AWG. The signals carried by the laser light may be provided by an OLT. Thus, multiple subscribers that are in the same geographical proximity can access the services of a service provider by each connecting to a different port of the AWG.
By the principle of reciprocity that applies to non-magnetic passive optical devices (e.g., the AWG), a signal of the proper wavelength may be injected to the given port of the AWG, whereupon it is multiplexed onto the single fiber. This provides an upstream path from the ONU to the OLT.
Conventional AWG-based WDM PON access networks have several inherent constraints that limit their ability to provide open access. Open access refers to the sharing of access network infrastructure among multiple service providers and/or multiple services offered by the same business entity. Each of the providers/services may offer a different format, speed, pricing, quality of experience, or other differentiating features. For business or technical reasons, it may not be feasible to provide all of these options from a single OLT. However, cost constraints at the OLT strongly motivate optical design in arrayed devices, wherein each transmitter (receiver) operates at a wavelength adjacent to that of its neighbor. The arrayed device thus produces a spectrum of transmit (receive) wavelengths. In a conventional network with an AWG in the field, a given transmitter (receiver) is mapped to a given ONU in a fixed relationship, as determined by the ONU's connectivity to an AWG port. As such, there is no flexibility point to assign OLT transmitters (receivers) to randomly-located ONUs.
An existing alternative solution proposes to separate each wavelength in the central office, and patch the proper wavelength to the proper OLT port either manually or through a reconfigurable optical add-drop multiplex (ROADM). This solution implies additional cost and complexity in the central office. Another possible solution could involve fully tunable OLT transmitters (receivers), but these may not be technically or economically feasible.